# c3_example.py
def get_age():
    while True:    
        str_age = input('please input your age\n')
        if str_age.isdigit() == True:
            age = int(str_age)
            return age
        else:
            print('age must be digit')
def panduan(age):
    if  age > 20:
        print('you are too old')
    else:
        print('you are so young')

# main   
age = get_age()
panduan(age)
